FWIW, Matthew and I know about the issue but just haven't found time
to fix it. The problem is specific to multi-line paren highlighting
(or other multi-line highlighting, but parenthesis highlighting is the
easiest to make it happen). The problem is that the interface
presented to the Scheme code for redrawing editors is fairly coarse
grained when you want to draw outside inside the editor canvas's
graphical region, but to the right of the widest line in the editor
(the only thing you can do to reliably draw there is redraw the entire
window).
On macs it is less visible because the drawing appears to be
significantly more efficient, or maybe just the windows machines are
slower, I don't konw. I certainly notice it too on my (sloooooow)
windows machine.
Not that that is much help, I know.
Robby